In situations such as vehicle collisions and explosions due to mines and IEDs, a chief cause of injury is the extreme impulses experienced; the human body has limited range of endurance for accelerations over durations of time. (See FIG. 9 for a chart of injury levels as function of acceleration in g (y-axis) vs. time (x-axis), [Eiband A. et al, 1959]). To lower the maximum impulse experienced, the distance through which the body travels when changing its initial to final velocity must be increased, or equivalently, the time during which the acceleration is experienced must be increased. For example, in a head-on collision of a vehicle with a rigid wall, the occupant's body will undergo a change from the car's initial speed to zero speed within a certain distance. The acceleration undergone is determined by the initial velocity and this distance. If this distance can be increased, the acceleration will be decreased. Care must be taken that the passengers will experience the maximum possible acceptable impulse or less, which can be accomplished by use of energy-absorbing elements of suitable design, devices to increase the travel available to the occupant, or both. The ideal energy absorber connecting a passenger to the rest of a vehicle transmits the maximum acceptable stress to the occupant or less, reaching this level after a minimum of travel. It would transmit this level of stress and no more, no matter the level of stress imparted to it. Solutions known from the prior art provide shock absorbing seats based on different types of elastic or plastic deformation or breakage of metallic components, collapsible bar mounts or columns made of metals and/or composite materials, crushable honeycomb, etc. Some available solutions present a full system including both an original seat and a built-in integrated absorbing mechanism.
For example U.S. Pat. No. 4,204,659 provides an energy absorber consisting of a conventional shock absorber in series with a rupturing diaphragm. However the stress-strain profile provided is specified to comprise an initial high peak, followed by a low valley, followed by a constant intermediate force level plateau [column 1 line 53]. It will be found that this profile is in fact suboptimal, as the ideal energy absorber reaches the maximum acceptable stress quickly and remains at this level for the total available travel. Furthermore the design relies on a conventional shock absorber and additional elements, which is a more complex design than needed for this application.
U.S. Pat. No. 5,131,470 discloses a single-event energy absorber designed for use with a so-called perforating gun in well bores. This energy absorber is designed to be deformed elastically when stressed past a certain amount, and to thereby absorb mechanical energy. Unlike a spring, the mechanical energy absorbed by such an element is released as heat and is not stored. It will be appreciated that such an energy absorber may be of use in systems designed for example to absorb shock in motor vehicle accidents. The energy absorber disclosed in '470 takes the form either of a cylinder coiled in helical fashion or a honeycomb matrix, both of which are intended to absorb energy in compression. It is the object of the current invention to absorb energy in tension, allowing for different configurations than possible for an element that absorbs energy in compression only. Furthermore the absorbers of '470 provide a certain fixed stress-strain profile which can be changed only by manufacturing elements of different parameters. It is an object of the current invention to provide a unit whose parameters are determined by a single cut introduced into the body of the device. This allows the unit to be produced in a single form in mass, and tailored to specific applications as needed.
U.S. Pat. No. 4,791,243 provides a coiled device intended for long-stroke plastic deformation and subsequent energy absorption. The device allows for a large deformation in comparison to the size of the device, e.g. the deformation may be 20 times the length of the device. It consists of a planar coiled element that stretches when subjected to a stress greater than a certain amount. However the device provides a certain fixed stress-strain profile which can be changed only by manufacturing elements of different parameters such as the planar thickness of the coil, the thickness of each turn, and the coil's outer diameter. In the applications mentioned for the device, namely the connection of electrical towers in such a way as to prevent the fall of one tower from pulling adjacent towers down, the exact stress-strain profile is more or less irrelevant, the main requirement being high deformation capability. In the case of a device intended to protect human beings in the case of crash or explosion, it is clear that the exact stress-strain curve is of paramount importance since the human body can withstand only a certain maximum stress without injury. To increase travel while still providing the same reaction force, the length of the spiral must be increased. This will therefore increase the outer diameter of the device. This increased outer diameter will increase the volume of the device. In applications where volume, height, and/or weight are limited the described patent will be at a disadvantage as compared to a device whose volume and weight does not increase to give increase travel. In the described application of strain relief for electrical towers the volume and weight of the device are largely irrelevant, but it will be appreciated that in aircraft or vehicles the allowable weight and volume of such a device will be limited.
U.S. Pat. No. 5,564,535 discloses a shock absorbing pad comprising a series of interconnected fluid reservoirs in the form of spheres partially filled with liquid. This device is designed to absorb a certain level of impact by forcing fluid from one sphere to the adjacent spheres, and for impact greater than this level to allow the spheres to rupture, thereby absorbing the shock. However it will be seen that the tunability of the stress-strain curve in this device is limited, when one considers that the ultimate stress the pad can provide is dependent upon the viscosity of the liquid within the spheres, which must take a value within a range generally far below that provided by solid materials. It is clear from the force-velocity curves provided [e.g. FIG. 5 of '535] that the ideal profile of rapidly reaching a plateau value just below the maximum acceptable force has not been attained. The device is designed to absorb energy in compression, preventing its use in applications where a tension member is necessary. Finally the planar nature of the device limits the maximum allowable travel, which in turn will limit the degree to which the device can reduce the accelerations experienced.
Similarly, U.S. Pat. No. 6,547,280 provides alternating front and rear projections, which absorb impact by plastic deformation such that the curve of stress to strain shows a plateau for example at a level of stress which does not break bone. However it will be seen that the tunability of the stress-strain curve in this device is limited, requiring manufacture of a sheet of different material or density of projections to change its stress-strain characteristics. The device is designed to absorb energy in compression, preventing its use in applications where a tension member is desired. Finally, the planar nature of the device limits the maximum allowable travel, which in turn will limit the degree to which the device can reduce the accelerations experienced.
U.S. Pat. No. 6,682,128 provides alternating ‘gamma’ and ‘delta’ structures, which absorb impact by some combination of elastic and plastic deformation. However it will be seen that the tunability of the stress-strain curve in this device is limited, requiring manufacture of a sheet of different material or depth of recess, depth of channels, inter-recess spacing, wall inclination, inter-module inclination, and/or recess shape to change its stress-strain characteristics. Furthermore the device is designed to absorb energy in compression, preventing its use in applications where a tension member is desired.
Hence it is the object of the current invention to fulfill the long felt need for a single-event energy absorber which absorbs energy in tension, which can provide a large ratio of deformation to initial size, and whose stress-strain characteristics can be tuned to those required for optimum safety performance by introduction of a single cut made into a mass-producible device.